This invention relates to the display of a pseudo-continuous tone image in a display device having only a limited number of discontinuous displayable colors.
It is possible to produce a display image (a pseudo-continuous tone image) which, though the number of colors that can be displayed is small, provides the effect as if colors of intermediate tones were also displayed, by utilizing the spatial integration characteristics of the sense of human sight. This can also be accomplished when a display device is employed in which the number of intensity levels for monochromic or black-and-white images is limited. The term "color" used herein means not only the colors in a multi-color display device but also the intensity levels of a monochromic or black-and-white display device.
A continuous tone can not be sufficiently displayed if the colors of an original image (an input image) are quantified at a predetermined threshold level. Therefore, in a pseudo-continuous tone display, the color of each picture element is compared with a threshold value which is calculated in accordance with a certain rule, in order to determine a display color. Such a method is generally referred to as the "dither" method. Various dither methods are known in the art. (For example, refer to "A Survey of Techniques for the Display of Continuous Tone Pictures on Bilevel Displays", Computer Graphics and Image Processing, May, 1976, pages 13-40.) Typical examples of these dither methods are an ordered dither method and an error diffusion method. In the ordered dither method, the threshold value for quantifying the original image is determined solely in accordance with the coordinate information of the picture element to be displayed. Generally, this method regards the picture surface as an aggregate of n.times.n sub-matrices, and uses as the coordinate information a value representing one side n of the matrix as module n. (Refer to the reference described above.)
Though the quantity of necessary calculation can be reduced by the ordered dither method, the method involves the problems in conjunction with the smoothness of the image and the spatial resolution. Another problem is that a particular pattern appears on the picture surface.
The error diffusion method, however, compensates for the error between the color of an original image (input image) in a certain picture element and the color that is most approximate to the former and can be displayed, by weighting and distributing not-determined picture elements (pixels) close to the original color as will be described later in further detail. Though fine, particular crease patterns appear on the picture surface, the error diffusion method provides satisfactory smoothness and spatial resolution. However, since the quantity of calculation is great, this method involves the problem of a long processing time (which is some dozens of multiples of the processing time of the ordered dither method). Furthermore, since the result of processing of each picture element affects sequentially the content of processing of not-determined pixels as will be described later, it is difficult to accomplish parallel processing to reduce the processing time. This is the most crucial problem with this method.